The present invention relates to a process and apparatus for forming single or multi-wall microcapsules.
There are a variety of processes for the formation of microcapsules as well as a variety of apparatus to make microcapsules. In the main, all are directed to the effort of rapidly forming microcapsules of substantially uniform particle sizes and with substantially uniform outer shells. While all such prior processes and apparatus are generally suitable to a certain degree, they none theless individually suffer from one or more defects such as being procedures which are costly to utilize, and/or ones in which it is difficult to control the uniformity of the size of the microcapsule formed, and/or cannot make the microcapsules in a rapid manner. This is due to the fact that it is difficult to obtain substantially uniformly sized microcapsules having a substantially complete coating and uniform coating thickness of the encapsulating material, whether there be a single outer shell or multiple outer shells to ensure the uniformity of the resultant microcapsules.
Such prior techniques include those as set forth in U.S. Pat. No. 3,423,489 wherein the fill material and the encapsulated material are extruded together through a device in which the fill material is extruded in the center of the device while at the same time therearound there is extruded the encapsulating material so that one has a tube of the coating material surrounding the inner core of the encapsulating material. This patent goes on to state the very precise controls that are required in order to be able to have the uniform particles and the reliance upon "natural forces" in order to form the individual droplets. Another procedure is that described in U.S. Pat. No. 3,015,128 in which a centrifugal nozzle is utilized causing the filling material to pass through a continuous flow of encapsulating material and on to a rotating distributing plate. The deficiencies of this process are discussed in columns 1 and 2 of the U.S. Pat. No. 3,423,489. Other devices, such as those disclosed in U.S. Pat. No. 3,241,520 and 4,217,851, rely on fluidization of the filler material at the same time applying a coating thereon. The length of time individual particles are in the fluidized condition and exposed to the coating material will vary widely so these processes suffer from the defect of not being able to get the uniformity of particle microcapsules desired.
Lastly, there are the processes, such as that set forth in U.S. Pat. No. 2,685,537, wherein again a spray of a coating material is utilized and in this case the requirement to have ionization rings in order to enable the tablets to be coated to be a different charge from the coating which is sprayed thereon.
None of these procedures, as noted, is entirely suitable to give the uniformity of outer shell size and uniformity of microcapsules. Also, none of these processes has the ability to form single or multi-wall microcapsules in the same system and a system which can form microcapsules that can vary in sizes from less than 100 microns to several thousand microns or larger.